Viacom wants to prove that YouTube's sole purpose back then was to capitalize on illegal content. The emails and IMs do seem to show that YouTube wanted illegal content on the site to make it more "viral" and prevent it from being an also-ran in the Web video race.

The emails and IMs also seem to show that Google knew about this plan when it bought YouTube for $1.65 billion in 2006.

The problem for Viacom is that YouTube also housed plenty of user-generated content, so it wasn't strictly about the illegal content.